The present invention relates to a printing apparatus for continuously printing a seamless continuous pattern on wall paper, corrugated cardboard, color wrapping paper, face steel plate, etc.
The conventional offset printing is performed by rotating in mutual contact a plate cylinder, a blanket cylinder and an impression drum. FIG. 1 shows a concept of sheet-fed offset printing machine. As shown in the drawing, the offset printing machine includes a plate cylinder 1, a blanket cylinder 2 and an impression cylinder 5. The plate cylinder 1 is formed of an aluminum cylinder having a thin aluminum sheet wound thereon as a printing plate. A photosensitive resin layer having a compatibility with an ink is formed on the thin aluminum sheet. A rubber blanket sheet (not shown) is attached to the blanket cylinder 2 by a metal fitting bar. An oil-resisting surface rubber layer for performing an ink transfer is formed on the rubber blanket sheet.
A groove 3 is formed on each of the plate cylinder 1 and the blanket cylinder 2, with the result that a blank portion in which the printing is not performed is formed on a printing medium 4 such as a paper sheet. The impression drum 5 serves to transfer in a predetermined direction the printing medium 4 held between the blanket cylinder 2 and the impression cylinder 5. An ink is supplied from an inking device 6 consisting of a plurality of rolls 6a, 6b, 6c, 6d, 6e and 6f onto the plate cylinder 1.
The rubber blanket for the conventional offset printing machine is prepared in a length of several hundred meters and, when used, the long rubber blanket is cut to meet a required machine size. In general, the rubber blanket is prepared by bonding 2 to 4 woven fabric sheets with a rubber paste, and the surface rubber layer is heated in a vulcanizer for the vulcanization. Then, the surface rubber layer is polished to have a required thickness, thickness uniformity and surface roughness.
FIG. 2 exemplifies an offset rotary press including a plurality of units. Each unit comprises a pair of blanket cylinders 12 having a web paper 14 held therebetween, plate cylinders 11 positioned in contact with the blanket cylinders 12, inking devices 13 for supplying an ink to these plate cylinders 11, and dampening units 15. Printing can be performed on both side of the web paper 14 simultaneously. If the paper sheet 14 is passed through four units continuously, four color printing can be performed on both surfaces of the paper sheet 14. The particular offset rotary press is widely used for the printing of news paper advertising paper sheets, etc.
A printing plate and a rubber blanket are mounted in the form of a sheet to each of the plate cylinders and the blanket cylinders in the case of the offset rotary press, too, with the result that a blank portion in which the printing is not performed is formed in a width of about 10 mm. Also, the thickness of the printing paper sheet used is limited to about 0.1 mm or less, making it impossible to carry out the printing on a corrugated cardboard or a steel plate.
FIG. 3 shows a concept of a flexographic press. The flexographic press comprises mainly an ink pan 21 housing an ink, a rubber roll 22 having a part thereof dipped in the ink housed in the ink pan 21, an anilox roller 23 having fine cells formed on the surface, a doctor blade 24 for removing an excess ink, an plate cylinder 26 positioned adjacent to the anilox roller 23 and having a flexographic printing plate 25 formed on the surface, and an impression cylinder 28. A printing medium 27 such as a corrugated cardboard or a plastic film is held between the plate cylinder 26 and the impression cylinder 28.
In the flexographic press, an ink is supplied from the rubber roll 22 to the anilox roller 23, and an excess ink is removed by the doctor blade 24 such that a required amount of the ink is supplied to the flexographic printing plate 25. Further, the ink is transferred from the flexographic printing plate 25 onto the printing medium 27 so as to finish the printing operation. In the flexographic printing, the printing plate is prepared by forming a manual or laser engraving on a soft and elastic material such as a rubber plate or a photosensitive resin plate. However, the flexographic printing is far inferior in the printed image quality to the offset printing.
The printing plate for the offset printing is prepared by coating an aluminum plate with a lipophilic (compatible with ink) photosensitive resin, followed by exposing the photosensitive resin layer to light through a negative film so as to fuse the non-image portion. As a result, the non-image portion is exposed to the outside so as to be made hydrophilic. The most excellent image quality can be obtained by the offset printing among the various printing techniques available nowadays partly because the image quality is determined by the magnitude and area ratio of the dot and partly because the ink is transferred to the printing medium such as a paper sheet through a rubber blanket. In the offset printing, the rubber blanket is said to be the most important factor for determining the quality of the printed image.
Recently, a blanket in which a porous rubber layer is interposed as a compressible layer between adjacent woven fabric sheets is mainly used in the printing field. The entire thickness of the blanket generally falls within a range of between 1 mm and 2 mm, and the actual thickness is determined to meet the specification of the printing machine.
In the ordinary offset printing machine, the printing is performed successively with inks of four colors, i.e., black, cyan, magenta and yellow, by four units so as to obtain a colored print. In the conventional offset printing machine, however, a groove is formed in each of the plate cylinder and the blanket cylinder for attaching a sheet-like printing plate and blanket to these cylinders. It follows that the size of the print is limited. For example, it is impossible to print a continuous pattern on a large and thick printing medium such as a wall paper sheet wrapping paper or a face steel plate.